linux-kernel - Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in fpa

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Date: Mon, 15 Apr 2024 18:58:30 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Mark Rutland <mark.rutland@....com>, Kees Cook <keescook@...omium.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        syzbot <syzbot+cb76c2983557a07cdb14@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [hardening?] [mm?] BUG: bad usercopy in fpa_set

On 2024/04/15 18:44, Russell King (Oracle) wrote:
> On Mon, Apr 15, 2024 at 06:38:33PM +0900, Tetsuo Handa wrote:
>> On 2024/04/15 18:02, Mark Rutland wrote:
>>>   08626a6056aad824 ("arm: Implement thread_struct whitelist for hardened usercopy")
>>>
>>> That commit says that all accesses are bounce-buffered and bypass the check,
>>> but AFAICT the fpa_set() code hasn't changed since then, so either that was
>>> wrong or the user_regset_copyin() code has changed.
>>
>> Then, can we go with https://lkml.kernel.org/r/0b49d91b-511f-449e-b7c3-93b2ccce6c49@I-love.SAKURA.ne.jp ?
> 
> Have you visited that URL? It doesn't point to an email containing a
> patch, so sorry, I don't know what patch you're referring to.
> 

Containing a link to a diff. ;-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index c421a899fc84..347611ae762f 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -583,10 +583,15 @@ static int fpa_set(struct task_struct *target,
 		   const void *kbuf, const void __user *ubuf)
 {
 	struct thread_info *thread = task_thread_info(target);
+	const unsigned int pos0 = pos;
+	char buf[sizeof(struct user_fp)];
+	int ret;
 
-	return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
-		&thread->fpstate,
-		0, sizeof(struct user_fp));
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
+				 buf, 0, sizeof(struct user_fp));
+	if (!ret)
+		memcpy(&thread->fpstate, buf, pos - pos0);
+	return ret;
 }
 
 #ifdef CONFIG_VFP